if you are concerned just move it away a bit. put some sticky tape near it so when you next open it you catch some. if they are the pest whitefly then yes they need killing.
if they are not 'white flies and there are many different species of them I suspect the flies are just fruit flies or the normal decomposer flies. in which case they wont do any harm.

Thank you Seaburngirl,

Is the picture an allotment, so immaculate!

Thank you Honeysuckle, we do our best. The greenhouse in the photo is made of recycle plastic bottles.

Do you have a lot of green waste in the form of cabbage, sprouts etc? I had exactly the same problem. My compost bin was situated near my greenhouse and when I removed the lid of the bin I had a cloud of white flies. If you grow cabbage family, then this could be a problem. Why not try and sprinkle the contents of your bin with a little sulphate of ammonia from time to time; this will help the contents to decompose more quickly, build up the heat and the acid environment might help to kill off the white fly population.

If you put a layer of lawn mowings or wood ash over the fresh stuff the flies will go.

Thank you Jimmytheone and Steragram. I will give a try with wood ash & sulphate.